Intervertebral disc prosthesis

ABSTRACT

Intervertebral disc prostheses are provided. More particularly, the invention provides disc prostheses that have adjustable disc heights, that are readily converted into fusion prostheses and that provide a range of motions that effectively mimic the natural range of motions of a spinal disc.

BACKGROUND

The present invention relates generally to the field of intervertebraldisc prostheses. More particularly, the invention relates to discprostheses that have adjustable disc heights, that are readily convertedinto fusion prostheses and that provide a range of motions thateffectively mimic the natural range of motions of a spinal disc.

Intervertebral discs provide elastic support upon compression betweenadjacent vertebrae in the spine. Damage to these discs, resulting fromdegeneration and wear, may produce mobility limitations, pain,discomfort and even paralysis. Conventional approaches to the treatmentof pathologic, degenerated or ruptured spinal discs include fusion ofadjacent vertebrae and disc replacement. In a spinal fusion procedure,the faulty disc may be removed and replaced with a mechanical cage whichmaintains the proper disc spacing and helps support the load imposed onthe spine. Ultimately, bone grows in and around the cage. The vertebraethat are involved in the fusion no longer take part in normal spinalflexing.

Disc replacement is an alternative approach to fusion. Many discprostheses have been proposed. Unfortunately, the proposed discprostheses suffer from practical limitations. Ideally, a disc prosthesiswill mimic the range of motion of a natural vertebral disc, includingside-to-side and front-to-back bending motions, compression in the axialdirection and rotation and translation between vertebrae. The discprosthesis should provide a proper disc height between vertebrae as wellas proper alignment of the spine. The disc prosthesis should also bebiocompatible, stable and durable. A typical disc prosthesis may beexpected to last 30 years or more at approximately 2 million cycles peryear.

Unfortunately, disc replacement carries significant risks. For example,distracting the vertebrae to insert or replace a disc prosthesis mayinjure the vertebrae, the vertebral endplates and the surroundingtissues and ligaments, including the spinal cord, nerve elements andblood vessels. This can be particularly problematic if the discprosthesis requires repeated replacement or when it is desirable toswitch from a disc prosthesis to a fusion prosthesis.

Thus, a need exists for a durable disc prosthesis that minimizes strainon the vertebrae, endplates and ligaments during insertion andreplacement while providing a natural disc height and range of motion.Also desirable, is a disc prosthesis that may be easily converted into afusion prosthesis while minimizing the risk of disc extrusion and itspotentially lethal complications.

SUMMARY

Intervertebral disc prostheses are provided. In some embodiments, thedisc prostheses have adjustable disc heights. In other embodiments, thedisc prostheses may be readily converted into fusion prostheses. Instill other embodiments, the disc prostheses provide superior axialloading capabilities. The disc prostheses may be implanted in thecervical, thoracic and lumbar regions of the spine and may be insertedvia a lateral or anterior approach. The prostheses may be designed tomimic some or all of the natural degrees of motion provided by a spinaldisc.

One aspect of the invention provides an intervertebral disc prosthesishaving an adjustable disc height. The prosthesis includes a first baseplate which has an exterior surface and an interior surface. A first cupwhich defines a first concave surface is disposed on the interiorsurface of the first base plate. The prosthesis further includes asecond base plate, also having an exterior surface and an interiorsurface. A second cup which defines a second concave surface is disposedon the interior surface of the second base plate. The first and secondbase plates are disposed opposite one another in a substantiallyparallel relation, such the first and second cups are disposed oppositeand facing one another. A disc insert having two opposing convexsurfaces is disposed between and in contact with the first and secondconcave surfaces of the first and second base plates to provide anarticulating joint. The disc prosthesis is characterized in that one orboth of the first and second cups is mounted to its base plate through avertically adjustable support.

In an alternative configuration, an intervertebral disc prosthesishaving an adjustable disc height, includes a first base plate having anexterior surface, an interior surface and a cup, which defines a concavesurface, disposed on its interior surface. The prosthesis furtherincludes a second base plate having an exterior surface, an interiorsurface and a knob, which defines a convex surface, disposed on itsinterior surface. The first and second base plates are disposed oppositeone another in a substantially parallel relation, such that the concavesurface of the cup and the convex surface of the knob are disposedfacing and in contact with one another to provide an articulating joint.The disc prosthesis is characterized in that one or both of the cup andknob is mounted to its base plate through a vertically adjustablesupport.

Another aspect of the invention provides a prosthesis assembly which maybe readily converted from a disc prosthesis into a fusion prosthesis insitu. The prosthesis assembly includes a first base plate characterizedby a circumferential edge, an interior surface and an exterior surface.A threaded groove extends into the circumferential edge along theinterior surface of this first base plate. The prosthesis assembly alsoincludes a second base plate characterized by a circumferential edge, aninterior surface and an exterior surface. A threaded groove extends intothe circumferential edge along the interior surface of the second baseplate. The first and second base plates are disposed opposite andsubstantially parallel to one another, such that the grooves on theirinterior surfaces are disposed opposite and facing one another. Athreaded rod which screws between the pair of oppositely disposedgrooves on the first and second base plates is also provided. When thethreaded rod is in place between the grooves, it immobilizes orpartially immobilizes the two base plates. The prosthesis assembly mayfurther include a joint disposed between the two base plates to provideone or more degrees of motion (e.g. rotating, bending, compression,translation) of a natural intervertebral disc. This joint may be removedprior to the insertion of the threaded rod to convert the assembly froma disc prosthesis into a fusion prosthesis. However, the removal of thejoint is not necessary.

Still another aspect of the invention provides a disc prosthesis withimproved axial loading. This disc prosthesis includes a first and asecond base plate, each characterized by an exterior surface and aninterior surface. A cup, which defines a concave surface, is disposed onthe interior surface of each base plate and the base plates are disposedopposite and substantially parallel to each other, such that the cupsare disposed opposite and facing one another. A disc insert having twoopposing convex surfaces is disposed between and in contact with thefirst and second concave surfaces of the first and second base plates toprovide an articulating joint. The disc insert is characterized in thatthe walls that form the two opposing convex surfaces have a plurality ofcompressible helical slits defined therein.

Yet another aspect of the invention provides a prosthetic vertebralassembly having a height that may be adjusted in situ. Such assembliesmay be used to replace one or more vertebra and their associatedintervertebral discs. In one basic embodiment the prosthetic vertebralassembly includes a prosthetic vertebral body composed of a basecharacterized by a superior end and an inferior end, wherein thesuperior end is disposed opposite the inferior end. A superiorvertically adjustable support is adjustably mounted to the superior endof the base and an inferior vertically adjustable support is mounted tothe inferior end of the base. A first intervertebral disc prosthesis ismounted to the superior adjustable support and a second intervertebraldisc prosthesis is mounted to the inferior adjustable support, such thatthe base and the adjustable supports are sandwiched between the discprostheses. In this configuration, the base and the superior andinferior adjustable supports form a prosthetic vertebra between twoprosthetic discs. When the prosthetic vertebral assembly is implanted inan intervertebral space, the first and second disc prostheses are incontact with a superior and an inferior vertebra respectively. In someembodiments, the prosthetic vertebral assembly may be designed toreplace more than on vertebra by linking multiple components together.For example, a prosthetic vertebral assembly may be composed of a firstdisc prosthesis mounted to the superior end of a first prostheticvertebral body, a second disc prosthesis mounted between the inferiorend of the first prosthetic vertebral body and the superior end of asecond prosthetic vertebral body and a third disc prosthesis mounted tothe inferior end of the second prosthetic vertebral body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front view of a cup adjustably mounted to a base platevia a threaded stem.

FIG. 2 shows a cross-sectional front view of the cup/stem/base plateassembly of FIG. 1.

FIG. 3 shows a top view of the cup/stem/base plate assembly of FIG. 1.

FIG. 4 shows a front view of the cup/stem/base plate assembly of FIG. 1,including a tab which locks in the height of the cup relative to thebase plate.

FIG. 5 shows a front view of the tab of FIG. 4.

FIG. 6 shows a cross-sectional side view of the tab of FIG. 4.

FIG. 7 shows a top view of the tab of FIG. 4.

FIG. 8 shows a cross-sectional side view of the cup/stem/base plateassembly of FIG. 4.

FIG. 9 shows a cross-sectional front view of a disc prosthesis,including the cup/stem/base plate assembly of FIGS. 1-4.

FIG. 10 shows a front view of a disc insert for a disc prosthesis.

FIG. 11 shows a front view of the disc prosthesis of FIG. 9 in placebetween two vertebrae.

FIG. 12 shows a front view of a disc prosthesis having two adjustablecups.

FIG. 13 shows a front view of a set screw that may be used to immobilizethe disc prosthesis of FIG. 12.

FIG. 14 shows a cross-sectional side view of the set screw of FIG. 13.

FIG. 15 a shows a front view of the disc prosthesis of FIG. 12 which hasbeen converted into a fusion prosthesis through immobilization with theset screw of FIGS. 13 and 14.

FIG. 15 b shows an enlarged view of a recoil wire that is used toprevent the set screw from becoming dislodged.

FIG. 15 c shows a cross-sectional side view of the fusion prosthesis ofFIG. 15 a implanted between two vertebrae.

FIG. 16 shows a bottom view of the superior cup/stem/base plate assemblyof the disc prosthesis of FIG. 12.

FIG. 17 shows a top view of the inferior cup/stem/base plate assembly ofthe disc prosthesis of FIG. 12.

FIG. 18 shows a cross-sectional side view of a set screw that may beused to immobilize the disc prosthesis of FIG. 12.

FIG. 19 a shows a front view of the disc prosthesis of FIG. 12 withoutthe disc insert. The base plates include screw holes to allow attachmentof a set screw.

FIG. 19 b shows a front view of the disc prosthesis of FIG. 19 a withthe set screw of FIG. 18 in place.

FIG. 20 shows a front view of a disc insert that may be used with thedisc prostheses of FIGS. 9 or 12.

FIG. 21 shows a cross-sectional view of a disc insert that may be usedwith the disc prostheses of FIGS. 9 or 12.

FIG. 22 shows a side view of the disc prosthesis of FIG. 12 whichincludes a pair of cables running substantially parallel between thebase plates.

FIG. 23 shows a side view of the disc prosthesis of FIG. 12 whichincludes a pair of cables criss-cross between the base plates

FIG. 24 shows a cross-sectional side view of a cup having a flat slitrunning through its concave surface. The cup is adjustably mounted to abase plate via a threaded stem.

FIG. 25 shows a front view of a prosthetic vertebral assembly implantedbetween a superior vertebra and an inferior vertebra.

FIG. 26 shows a cross-sectional front view of the prosthetic vertebralassembly of FIG. 25.

FIG. 27 shows a cross-sectional view of the prosthetic vertebral body ofthe prosthetic vertebral assembly of FIG. 25.

FIG. 28 shows a front view of the prosthetic vertebral body of FIG. 27.

DETAILED DESCRIPTION

One aspect of the invention provides intervertebral disc prostheses thatdo not require distraction or which minimize the degree of distractionrequired for their implantation. This provides an advantage over otherpresently known disc prostheses which require the vertebrae to bestretched further apart than their natural spacing in order to insert aprosthesis in the intervertebral space. This procedure presents anincreased risk of injury to the vertebra, the vertebral endplates andthe surrounding tissues and ligaments, including the spinal cord, nerveelements and blood vessels. In addition, implantation of conventionaldisc prostheses may require an asymmetric distraction of the vertebrae.During an asymmetric distraction the vertebrae rotate and collapsetoward one another at the side opposite the distracting side. The discprostheses provided by the present invention minimizes these risks byproviding a disc prosthesis that may be inserted between two vertebraewithout distraction or with minimal symmetric distraction and thenexpanded in situ. To the extent these prostheses distract the spine,they do so in a symmetric fashion without rotation or collapse of thevertebrae. This approach is advantageous because it minimizes facetloading. Additionally, because the disc height of these intervertebraldisc prostheses may be adjusted in situ, the prostheses may be tailoredto provide a desired disc height for a particular patient. Theseintervertebral disc prostheses are based on an articulating joint thatuses a ball-in-socket type mechanism where the joint may be expandedvertically in situ after the insertion of the prosthesis in theintervertebral space. Typically, the intervertebral disc prosthesis willinclude a means for simulating one or more degrees of motion of anatural intervertebral disc and a means for adjusting the discprosthesis height in situ. The means for simulating one or more degreesof motion of a natural intervertebral disc may be a joint and the meansfor adjusting the disc prosthesis height in situ may be a verticallyadjustable support.

In one embodiment, the disc prosthesis has an articulating joint thatincludes a single articulating interface defined by a concave surfaceand a complementary convex surface that fits into and articulates withthe concave surface. For the purposes of this disclosure, a part of adisc prosthesis that defines a concave surface will be referred to as a“cup” and a part of a disc prosthesis that defines the complementaryconvex surface will be referred to as a “knob.” At least one of the cupor the knob that define the articulating joint is mounted to a firstbase plate through a vertically adjustable support. The remaining knobor cup may be mounted to a second base plate through a verticallyadjustable support, fixedly mounted to a second base plate or may simplybe defined by a protrusion or indentation in a second base plate. Forthe purposes of this invention, a cup or knob is “disposed on” a baseplate if it is adjustably or fixedly mounted to the base plate, or if itis defined by the surface of the base plate itself. When the discprosthesis is in place in the intervertebral space, the first base plateand the second base plate are disposed opposite one another such thatthe concave and convex surfaces come together to form the articulatingjoint. Once the disc prosthesis is in place in the intervertebral space,the vertically adjustable support or supports may be adjusted to expandthe disc height until the natural disc height is restored and the baseplates are pressed more firmly against the vertebrae, stabilizing theprosthesis and minimizing the risk of disc extrusion.

In another embodiment, the disc prosthesis has an articulating jointthat includes two articulating interfaces defined by two opposing cupsseparated by a disc insert having two oppositely disposed convexsurfaces that fit into and articulate with the concave surfaces of thecups. In this design, one or both of the cups is mounted to a base platethrough a vertically adjustable support. When only one cup is somounted, the other may be fixedly mounted to a second base plate or maysimply be defined by an indentation in the second base plate. When thedisc prosthesis is in place in the intervertebral space, the first baseplate, the second base plate and their corresponding first and secondcups are disposed opposite one another with the disc insert disposedbetween the two cups, such that the convex surfaces of the disc insertand the concave surfaces of the cups come together to form thearticulating joint. Once the disc prosthesis is in place in theintervertebral space, the vertically adjustable support or supports maybe adjusted to expand the disc height until the natural disc height isrestored and the base plates are forced more firmly against the superiorand inferior vertebrae, stabilizing the prosthesis and minimizing therisk of disc extrusion.

The vertically adjustable support may be any support that can beadjusted in situ to change the spacing between the cup or knob mountedthereon and the base plate. As used herein, “vertically adjustable”indicates that the support may be adjusted vertically with respect tothe plane of the base plate.

In one exemplary embodiment, the vertically adjustable support iscomposed of a threaded stem that screws into a complimentarily tappedbore in a base plate. The bore may extend into or through the baseplate. Using this construction, the disc height of the prosthesis may beincreased or decreased by rotating the stem within the bore in onedirection or the other. The adjustable support and/or threaded stem mayoptionally include a stop which prevents the disc height from changingonce the support has been adjusted.

In one variation on the above-described embodiment, the verticallyadjustable support is a stem onto which a cup or knob is mounted. Anaxial tapped bore extends into the stem that screws onto a threaded stemextending from the base plate. Using this construction, the disc heightof the prosthesis may be increased or decreased by rotating the tappedbore on the threaded stem in one direction or the other. The adjustablesupport may optionally include a stop which prevents the disc heightfrom changing once the support has been adjusted.

In another design, the adjustable support may be a stem having asaw-tooth-like pattern along its edge. In this design, the ratchetingteeth along the stem engage with reciprocating teeth in a bore in thebase plate allowing the cup or knob mounted to the adjustable support tobe jacked up from the base plate in a step-wise fashion. Yet anotherdesign combines a screw-type rotational motion and a ratchetingmechanism. In this embodiment a threaded stem screws out of a tappedbore in the base plate and at regular intervals ratchets along the stemengage reciprocating teach in the bore to prevent unintended retraction.

Other suitable vertically adjustable supports include, but are notlimited to, those based on a cam mechanism, such as that described inU.S. Pat. No. 4,863,476 and those based on an expandable/collapsiblebellows, such as that described in U.S. Pat. No. 6,375,682.

The adjustable support and the cup or knob mounted thereon aredesirably, but not necessarily, removably attached to the underlyingbase plate such that they may be completely detached from the discprosthesis. This allows the cup or knob to be replaced when anarticulating joint begins to wear out or fails. Alternatively, theremoval of the articulating joint allows the disc prosthesis to beconverted into a fusion prosthesis. This may be accomplished byinserting a cage between the base plates to immobilize the base platesbetween the vertebrae. In the embodiments provided above, the adjustablesupport could be removed from its base plate simply by unscrewing thethreaded stem from its tapped bore or by ratcheting the saw-toothed stemout of its bore.

The base plates to which the adjustable supports are anchored may bedesigned to provide multiple sites of attachment for the adjustablesupports. For example, a base plate may include more than one bore alongits anterior-posterior axis for receiving a threaded stem. By changingthe location of the adjustable supports on the base plates, theplacement of the articulating joint and its center of articulation maybe tailored to meet the specific needs of a given patient.

The base plates may be made of any suitable biocompatible material andmay optionally be made of or coated with a porous material to allow boneand/or tissue growth therethrough. Alternatively, the base plates may bemade of a fenestrated biocompatible material that allows bone and/ortissue growth therethrough. Suitable biocompatible materials include,but are not limited to, metals such as titanium, titanium alloys, chromecobalt, or stainless steel. Other biocompatible materials include, butare not limited to, graphite and ceramics, such as hydroxapatites.Plastics may also be employed. Suitable plastics include, but are notlimited to, polyethylene (e.g. ultra high molecular weight polyethylene)and polyether ester ketone.

The exterior surfaces of the base plates (i.e., the surfaces that areadapted to be attached to the superior and inferior vertebrae definingthe intervertebral space) may be flat, but are desirably convex, suchthat they match the natural contours of the vertebral endplates. Theexterior surfaces and interior surfaces (i.e. the surfaces that face theintervertebral space when the disc prosthesis is in place) of the baseplates may be substantially parallel or may define a small angle (e.g.,less than about 10 degrees), providing a wedge shaped plate. Thecircumferential shape of the base plates is not critical, but should bechosen to provide a stable foundation for the disc prosthesis againstthe vertebral endplates. As such, the base plates desirably cover theendplates of the vertebrae substantially completely in order to avoidthe application of pressure to and the puncturing of the softer tissuein the nucleus of the endplates. In some instances the base plates mayhave a oval circumference. In other instances the base plates may have akidney-like circumference that mimics the natural circumferential shapeof the vertebrae. The base plates may be anchored to the vertebralendplates through any suitable attachment means, many of which are wellknown. For example, the base plates may be fastened to their respectiveendplates through bone screws, pins, pegs, teeth and the like.

The circumferential shape of the concave or convex surfaces that aredefined by the knobs or cups may take on a variety of shapes, includingcircular or ellipsoidal. An ellipsoidal shape is advantageous becausesuch a shape limits axial rotation without constraining flexion andextension or lateral bending in the direction parallel to the short axisof the ellipsoid. When the concave surfaces have an ellipsoidalcircumferential shape, the long axis of the ellipsoid may run parallelto the anterior-posterior axis of the base plate, perpendicular to theanterior-posterior axis of the base plate, or at an angle betweenparallel and perpendicular.

In some embodiments, the concave surfaces will include a flat stripdividing the apex of the concavity. When this concave surface engages acomplimentary convex surface, the strip allows for translation of theconvex surface along the flat strip of the concave surface, as well asrotation. By providing for translation, excessive stresses on the discprosthesis may be alleviated. The flat strip preferably runs along theconcave surface in a direction that is substantially perpendicular tothe anterior-posterior axis of the prosthesis, however otherorientations are possible.

The cups and/or knob may be positioned on the base plates such that thevertical axes through the centers of their concave or convex surfacescoincide with the midpoint of the anterior-posterior axes of the baseplates. Alternatively, the vertical axes through the centers of theconcave or convex surfaces may be displaced posteriorly with respect tothe midpoints of the anterior-posterior axes of the base plates. Thelatter embodiment may be advantageous because it more accuratelyreproduces the natural center of articulation of the spine.

Like the base plates, the disc inserts may be made of any suitablebiocompatible material, including those listed above. In someembodiments, the disc insert is desirably made from a plastic, such aspolyethylene. The circumferential shape of the oppositely disposedconvex surfaces will reflect the circumferential shape of the concavesurfaces. Thus, in some embodiments the circumferential shape of theconvex surfaces will be circular or ellipsoidal. The opposing surfacesare desirably, but not necessarily, bilaterally symmetric. A radioopaquemarker may optionally be incorporated into the disc insert to facilitatex-ray detection of the insert. For example, the disc insert may have aring made from a radioopaque material disposed in a groove around thecircumference of the insert. Alternatively, the disc insert may have aplate made from a radioopaque material disposed laterally through itscentral portion.

In order to provide for axial loading of the disc prosthesis, the discinsert may optionally be a compressible insert. For example, the discinsert may define one or more compressible slits around at least aportion of the periphery of its external surface in order to provide foraxial loading. The disc insert may optionally include a central collarseparating the two oppositely disposed convex surfaces. The collar mayinclude a flat ring around the periphery of the disc insert, whichallows for translation between the convex and concave surfaces, and anouter circumferential wall.

A second aspect of the invention provides an intervertebral prosthesisassembly that may be readily converted from a disc prosthesis into afusion prosthesis. These assemblies make it possible for a physician tochange the approach for treating back pain and disc degeneration from areconstruction or replacement of the degenerated joint to a spinalfixation and fusion using a single assembly. The assemblies areconverted from a disc prosthesis into a fusion through theimmobilization of the disc prosthesis in situ and may be used in theevent of a disc failure.

The basic features of the assembly include two base plates, each adaptedto be fixed to one of two vertebrae that define an intervertebral space.The base plates each have an circumferential edge, an exterior surfaceand an interior surface. When the prosthesis is inserted into theintervertebral space, the two base plates are disposed opposite oneanother. In this configuration, one base plate is fastened to thesuperior vertebral endplate and is referred to as the superior baseplate. The other base plate is fastened to the inferior vertebralendplate and is referred to as the inferior base plate. The superiorbase plate has at least one threaded groove extending into itscircumferential edge along its interior surface. Similarly, the inferiorbase plate has at least one threaded groove extending into itscircumferential edge along its interior surface. The grooves on theopposing plates are positioned such that they are disposed opposite andfacing one another when the prosthesis is in place in the intervertebralspace. In order to ensure that the base plates have the correctalignment when they are inserted, they may include some sort of marking(e.g. a line etched in the circumferential edges) that lines up when thebase plates are correctly positioned. The threads in the grooves areadapted to engage with a threaded rod such that the rod may be screwedinto place between the grooves to prevent the prosthesis fromarticulating. In this configuration, the rod serves as a cage in thefusion prosthesis. It is advantageous to provide as large a cage aspossible in the fusion prosthesis, therefore, it is desirable for thethreaded groove and the cage (i.e. threaded rod) to extend into theprosthesis assembly as far as possible. For example, if the prosthesisassembly includes a ball-in-socket type joint, the groove may extend tothe cups and/or knobs that form the joint.

The two base plates may include a single pair of oppositely disposedgrooves or may include two or more pairs of oppositely disposed grooveslocated at different positions along their circumferential edges. Forexample, the circumference of the base plates may be characterized ashaving a ventral edge (i.e. a portion of the circumferential edge thatfaces anteriorly when the prosthesis is in place in an intervertebralspace), a dorsal edge (i.e. a portion of the circumferential edge thatfaces posteriorly when the prosthesis is in place in an intervertebralspace) and a first and second lateral edge (i.e. portions of thecircumferential edge that face laterally when the prosthesis is in placein an intervertebral space). In some embodiments, the base plates willdefine a single pair of opposing threaded grooves located along theventral or lateral edges of the base plates. In other embodiments, thebase plates will each define two or more pairs of opposing threadedgrooves located along their ventral or lateral edges. In still otherembodiments, multiple pairs of opposing threaded grooves may be locatedalong a single portion of the circumferential edge (e.g. ventral orlateral). It should be understood that the different portions (ventral,dorsal and lateral) of the circumferential edge of a base plate may notbe rigidly defined, depending upon the shape of the base plate. However,the term “ventral edge” may generally encompass any portion of thecircumferential edge that is accessible from an anterior approach, theterm “lateral edge” may generally encompass any portion of thecircumferential edge that is accessible from a lateral approach and theterm “dorsal edge” may generally encompass any portion of thecircumferential edge that is accessible from a posterior approach.

The rod desirably has an outside diameter that is large enough todistract the vertebrae sufficiently to press the base plates snuglyagainst the vertebrae, stabilizing the fusion prosthesis and preventingthe base plates from separating further during use. Again, it should benoted that the term “rod” as used herein is not intended to denote onlya solid cylinder. The cylinder may be hollow. The threaded rod desirablyhas a tapered leading edge. This is advantageous because it allows thethreaded rod to be introduced into the cavity defined by the opposinggrooves without first having to distract the vertebrae. In thisconfiguration, the threaded rod passes into the cavity until it engagesthe threads in the opposing grooves. Once the threads have been engaged,the rod may be screwed between the grooves, causing the superior andinferior base plates to distract.

The base plates and the threaded rod may be made of any suitablebiocompatible material and are desirably made of or coated with a porousmaterial or of a fenestrated biocompatible material which allows boneand/or tissue growth therethrough.

When the prosthesis assembly is acting as a disc prosthesis, it willinclude a joint sandwiched between the two base plates. The joint may beany mechanism that simulates one or more of the natural degrees ofmotion of the spinal column. Various types of joints for providingdegrees of motion are known. These include ball-in-socket mechanismsmade from complementary concave and convex surfaces that form anarticulating joint between two opposing base plates. Other discprosthesis include a flexible rubber or polymeric insert disposedbetween two base plates to replicate natural spinal motion. Still otherdisc prosthesis include mechanical damping mechanisms, such as springs,disposed between opposing base plates in order to mimic natural spinalmotion. Any of these joints which replicate one or more degrees ofspinal motion may be utilized in the prosthesis assemblies providedherein. In some embodiments the means for providing motion, such as anarticulating ball-in-socket type joint, is offset posteriorly withrespect to the anterior-posterior axes of the base plates. This designmore accurately simulates the position of the natural center ofarticulation and provides more space for and easier access to a pair ofopposing threaded grooves along the ventral edge of the base plates.

The joint is desirably, but not necessarily, removable, such that it maybe removed prior to the insertion of the threaded rod which converts thedisc prosthesis into a fusion prosthesis. For example, when a discprosthesis having an articulating joint composed of a disc insertdisposed between two cups, as described in detail above, is used, thedisc insert and the cups may be removed prior to immobilizing the baseplates. Alternatively, the insert could be immobilized by rigidlyattaching it to the prosthesis assembly. In one such configuration, thedisc insert may include a tapped bore into which a screw may be insertedto fasten the insert to the threaded rod.

The range of rotational motion provided by the disc prostheses mayoptionally be restricted in order to provide more natural disc-likemovement. This may be accomplished by securing one or more cablesbetween the superior and inferior base plates such that the cablesprevent unrestricted rotation of one plate with respect to the other.One or more cables may be used and they may be attached to the baseplates at a variety of locations. The cables are preferably attached tothe circumferential edge of the base plates. When multiple cables areused, neighboring cables may be attached between the base plates in asubstantially parallel relation or they may be attached such that theycriss-cross. The cables may be attached to the base plates by anysuitable means, such as with welds, hooks, pins, snaps, and the like.The cables may be rigidly or removably fixed to the base plates. Thelatter embodiment is advantageous because it allows the cables to bemoved out of the way in order to make adjustments to the prosthesis. Thecables may be made of any biocompatible material that is sufficientlyelastic to provide a limited degree of rotational motion. In oneembodiment, the cables are made from a memory metal alloy that exhibitssuper-elastic properties at body temperature. A discussion of suitablebiocompatible memory metal alloys may be found in U.S. patentapplication Publication No. 2003/0009223, which is incorporated hereinby reference. Stainless steel is another example of a suitable materialsfrom which the cables may be made.

Another aspect of the invention provides a prosthetic vertebral assemblythat may be used to replace one or more vertebrae and the intervertebraldiscs associated therewith. The height of the assemblies may bevertically adjusted, that is adjusted in the direction along the longaxis of the spinal column when the assemblies are implanted. Thisfeature makes it easy to tailor the assembly height to a particularpatient and to adjust the height of in situ if necessary. The assembliesinclude at least one vertically adjustable prosthetic vertebral bodythat is made from a base having a superior end and an inferior end,where the term “superior end” refers to the end of the body that faces asuperior vertebra when the assembly is implanted in a spine and the term“inferior end” refers to the end of the body that faces an inferiorvertebra when the assembly is implanted in a spine. A superiorvertically adjustable support is adjustably mounted to the superior endof the base and an inferior vertically adjustable support is adjustablymounted to the inferior end of the base. A first intervertebral discprosthesis is attached to the superior vertically adjustable support anda second intervertebral disc prosthesis is attached to the inferiorvertically adjustable support, such that the prosthetic vertebral bodyis sandwiched between the first and second disc prosthesis in agenerally axial alignment. The prosthetic vertebral body may optionallybe adapted to accept screws, or other attachments means, that wouldpermit the prosthetic vertebral body to accept a stabilizing device forstabilizing the prosthetic vertebral assembly in a patient's spine.

The vertically adjustable supports may be any supports that may beadjusted in situ to change the overall height of the prostheticvertebral body. In one exemplary embodiment, the base of the prostheticvertebral body comprises a threaded rod characterized by a superior endand an inferior end and the first and second vertically adjustablesupports each define a tapped bore, extending into one surface thereof,which screws onto one end of the threaded rod. In this design the heightof the prosthetic vertebral body may be increased or decreased byrotating one or both of the supports on the threaded rod in onedirection or the other. The adjustable supports and/or the threaded rodmay optionally include a stop which prevents undesired expansion orcontraction of the prosthetic body height once it has been properlyadjusted.

In another embodiment, the base of the prosthetic vertebral body definesa tapped bore extending through the base, or two oppositely disposedtapped bores extending into opposing sides of the base, and the superiorand inferior vertically adjustable supports are threaded rods adapted toscrew into opposite ends of the tapped bore or into oppositely disposedtapped bores in the prosthetic vertebral body. Using this construction,the height of the prosthetic vertebral body may be increased ordecreased by rotating the threaded rods in the tapped bore or bores inone direction or the other. The adjustable supports and/or the tappedbore(s) may optionally include a stop which prevents undesired expansionor contraction of the prosthetic body height once it has been properlyadjusted.

Alternatively, the base of the prosthetic vertebral body may comprise astem having a saw-tooth-like pattern along its periphery at one end andan opposing saw-tooth like pattern along its periphery at the opposingend. In this design, the ratcheting teeth along one end of the stemengage with reciprocating teeth in a bore defined by the superiorsupport and the ratcheting teeth along the opposing end of the stemengage with reciprocating teeth in a bore defined by the inferiorsupport, such that the height of the prosthetic vertebral body may beincreased by jacking up one or both supports on the stem. Yet anotherdesign combines a screw-type rotational motion and a ratchetingmechanism. In this embodiment the base of the prosthetic vertebral bodyis a threaded stem having ratchets at regular intervals that engagereciprocating teeth in the tapped bores of the adjustable supports toprevent unintended contraction of the prosthetic body height.

The base and the superior and inferior vertically adjustable supportsmay be made of any suitable biocompatible material. Suitablebiocompatible materials include, but are not limited to, metals such astitanium, titanium alloys, chrome cobalt, or stainless steel. Otherbiocompatible materials include, but are not limited to, graphite andceramics, such as hydroxapatites. Plastics may also be employed.Suitable plastics include, but are not limited to, polyethylene (e.g.ultra high molecular weight polyethylene) and polyether ester ketone.

The dimensions (e.g. lateral and anterior-posterior widths) of theprosthetic vertebral body are desirably designed to mimic those of anatural vertebra. The prosthetic vertebral body, and the superior andinferior vertically adjustable supports in particular, may have avariety of circumferential shapes, however, the circumferential shapepreferably will be generally cylindrical.

The first and second intervertebral disc prostheses that are mounted toopposing ends of the prosthetic vertebral body may have a variety ofdesigns, provided they are adapted to be mounted to the vertebral bodyin a configuration and alignment that allows them to replace a naturalintervertebral disc when the prosthetic vertebral assembly is implantedinto a patient's spine. The disc prostheses may be mounted to theprosthetic vertebral body by any suitable means, including but notlimited to, screws, pins, welds, and the like. In some embodiments thedisc prostheses may be mounted to the prosthetic body by allowing one orboth of the vertically adjustable supports serve as a part of the discprostheses.

Typically, a suitable intervertebral disc prosthesis will include ajoint sandwiched between a superior base plate and an inferior baseplate. The joint may by any mechanism that simulates one or more of thenatural degrees of motion of a spinal column. Examples of suitablejoints include, but are not limited to, those based on a ball-in-sockettype interface, those based on a flexible rubber or polymeric insert andthose based on a mechanical damping mechanism, such as a spring. In someembodiments, the superior and/or inferior vertically adjustable supportswill themselves provide base plates for the disc prostheses.

One or both of the intervertebral disc prostheses of the prostheticvertebral assembly may be an intervertebral disc prosthesis having anadjustable disc height. Such vertically adjustable disc prostheses arediscussed in detail above. The combination of a prosthetic vertebralbody having a vertically adjustable body height with one or more discprostheses having vertically adjustable disc heights is advantageousbecause it provides the surgeon with a great deal of flexibility intailoring the prosthetic vertebral assembly for a given patient.

In some instances several intervertebral disc prostheses may be combinedwith two or more prosthetic vertebral bodies in order to replace entireportions of a patient's spine. For example, an assembly could becomposed of a first prosthetic vertebral body sandwiched between a firstand a second intervertebral disc prosthesis and a second vertebral bodysandwiched between the second and a third intervertebral discprosthesis.

The disc prostheses and prosthetic assemblies provided herein may befurther illustrated by the non-limiting embodiments discussed below inconnection with the figures. However, these embodiments are intendedonly to exemplify the invention and should not be construed to limit theinvention to any particular embodiment. The drawings are not necessarilyto scale and the relative dimensions of the disc prostheses andprosthesis assemblies provided therein may deviate from those shown inthe figures.

FIG. 1 shows a front view of an assembly including a cup adjustablymounted to a base plate in accordance with one embodiment of the presentinvention. FIG. 2 shows a cross-sectional view of the assembly of FIG. 1and FIG. 3 shows a top view of the assembly of FIG. 1. The assemblyincludes a cup 100 mounted on a threaded stem 102 which screws into atapped bore 104 in a base plate 106. The cup defines a concave surface108 and has a plurality of holes 110 disposed around its circumferentialedge 111. These holes are adapted to engage with a tool that may be usedto rotate the cup in situ. In FIG. 1, the cup further includes fourequi-spaced notches 112 cut into the periphery of its circumferentialedge, however, a different number of notches and different notchplacements are possible. A plurality of tapped holes 116 extend radiallyinto the threaded stem. As the threaded stem is rotated from its lowestposition upward, the tapped holes in the stem become exposed above theinterior surface 118 of the base plate 106. The tapped holes arevertically displaced from one another around the circumference of thethreaded stem, such that more tapped holes become exposed as thethreaded stem is rotated upward. For example, the tapped holes may bedisplaced such that one additional tapped hole becomes exposed everytime the threaded stem is raised an additional 1 millimeter in height.However, other displacements are also possible. The base plate includesat least one notch 120 that may be disposed opposite and facing a notchon the circumferential edge of the cup. The external surface 122 of thebase plate includes a plurality of pins 124 adapted to attached the baseplate to the endplate of a vertebra.

FIG. 4 shows how the notches on the cup and the base plate may be usedto lock in the height of the cup above the base plate. When the notch120 on the base plate 106 is lined up opposite a notch 112 on thecircumferential edge 111 of the cup 100, the two notches form a frameinto which a tab 126 may be fastened against the threaded stem 102 usinga screw 128 that engages one of the tapped holes 116 extending radiallyinto the stem. When the tab is in place the threaded stem is unable torotate. FIGS. 5-7 show a exemplary tab that may be used to lock in theheight of the cup in FIG. 4. FIG. 5 is a front view of the tab 126 andincludes a front view of a screw 128 extending through the tab andadapted to fit a tapped hole 116 in the threaded stem 102. FIG. 6 showsa cross-sectional side view of the tab and screw. As shown in figure,the screw may be aligned at a substantially right angle with respect tothe long axis of the tab (solid line), or may be aligned at a differentangle (dotted lines) to make it more accessible in situ. FIG. 7 shows atop view of the tab and screw. Here the contour of the inner surface ofthe tab matches the contour of the outer surface of the threaded rod toprovide a snug fit when the tab is screwed into place. Although the taband the frame in FIG. 4 are generally rectangular in shape, it should beunderstood that a variety of alternative shapes may also be employed.FIG. 8 shows a cross-sectional side view of the tab of FIGS. 5-7inserted into the assembly of FIG. 1. As illustrated in FIG. 8, when thetab is screwed in at an angle, the notches on the cup and the base platethat form the frame, should also be cut at an angle.

It is also possible to lock the cup against the base plate when the cupis in its fully contracted position. This may be accomplished byfastening an appropriately sized tab into the frame formed by twoopposing notches when the cup is resting against the base plate. In oneembodiment, the tab is fastened into place by screwing it directly tothe base plate itself. In this embodiment, the tab includes a screwaperture through which a screw may be inserted. The screw may then bescrewed into a tapped hole in the base plate, fixing the tab in theframe.

It should be noted that although the assembly of FIGS. 1-4 and 8 referto a disc prosthesis where a cup is mounted to a vertically adjustablesupport, an analogous design could also be used where a knob is mountedto a vertically adjustable support.

A cross-sectional front view of one exemplary disc prosthesis thatincludes the assembly of FIG. 1 is shown in FIG. 9. The disc prosthesisincludes a superior base plate 130 defining a first concave surface 132,a disc insert 134 having two oppositely disposed convex surfaces 136,138, and the inferior base plate 106 and cup 100 which provides a secondconcave surface 108. As discussed above, the cup is mounted on athreaded stem 102 which screws into a tapped bore 104 in the inferiorbase plate to provide a vertically adjustable support. The externalsurfaces 140, 122 of the superior and inferior base plates include pins124 to anchor the base plates to vertebral endplates.

FIG. 10 shows a front view of the disc insert of the disc prosthesis ofFIG. 9. As shown in the figure, the insert includes a collar 146 aroundits midsection. This collar has an upper rim 148 and a lower rim 150separated by a circumferential groove 152 which is adapted to receive aradioopaque ring (not shown) in order to locate the disc prosthesis insitu via x-ray imaging.

FIG. 11 shows the disc prosthesis of FIG. 9 inserted into theintervertebral space between a superior vertebra 142 and an inferiorvertebra 144. In the figure, the tab 128 that locks in the height of thecup 100 relative to the inferior base plate 106 faces in an anteriordirection which makes it more easily accessible in situ.

A prosthesis assembly that may be converted from a disc prosthesis intoa fusion prosthesis is presented in FIG. 12. This assembly is based onan articulating joint of the type shown in FIG. 9. Unlike the discprosthesis of FIG. 9, however, both cups 100, 154 of prosthesis of FIG.12 are vertically adjustably mounted to their respective base plates106, 130 on threaded stems 102, 156. The superior base plate 130 has anexterior surface 140 adapted to be attached to a vertebral endplatethrough a plurality of pins 124, an interior surface 158 that faces intothe intervertebral space when the disc prosthesis is in place and acircumferential edge 160. Similarly, the inferior base plate 106 has anexterior surface 122 adapted to be attached to a vertebral endplatethrough a plurality of pins 124, an interior surface 118 that faces intothe intervertebral space when the disc prosthesis is in place and acircumferential edge 162. A first threaded groove 164 extends into thecircumferential edge of the superior base plate along its interiorsurface and a second threaded groove 166 extends into thecircumferential edge of the inferior base plate along its interiorsurface.

The threading in the grooves is best seen in FIGS. 16 and 17. FIG. 16shows the view looking up at the superior base plate 130. FIG. 17 showsthe view looking down on the inferior base plate 106. In the embodimentdepicted in FIGS. 16 and 17, both base plates include two threadedgrooves, one at the ventral edge 164, 166 and one at the lateral edge168, 170.

It should be noted that the vertically adjustable cups shown in FIG. 12could also include the notch/tab configuration of FIG. 4 in order tolock in the height of the cups relative to the base plates, althoughsuch a configuration is not explicitly shown in FIG. 12.

In order to convert the prosthesis assembly of FIG. 12 into a fusionprosthesis, the base plates may be immobilized by screwing a threadedrod, such as a set screw 172, between opposing grooves in the superiorand inferior base plates. FIGS. 13 and 14 depict a front andcross-sectional side view, respectively, of a suitable set screw havinga tapered leading edge 173. FIGS. 15 a and c show a front view and across-sectional side view, respectively, of the prosthesis assembly ofFIG. 12. In FIG. 15 c the prosthesis assembly is implanted between asuperior vertebra 142 and an inferior vertebra 144. The prosthesisassembly shown in FIGS. 15 a and c includes the set screw 172 of FIGS.13 and 14 in place between the two threaded grooves 164, 166. In theembodiment shown in FIGS. 15 a and c, the disc insert 134 was removedprior to insertion of the set screw. However, it is possible to leavethe disc insert in place. Also, it should be noted that the cups in FIG.12 could also have been removed (i.e. unscrewed) or lowered prior toinserting the set screw. Although both cups are adjustably mounted tothe base plates in FIGS. 12 and 15, it is also possible for one or bothof the cups to be fixedly mounted to or simply defined by the interiorsurface of the base plates. The prosthesis assembly of FIG. 12 alsoincludes two “stops.” The first stop 174 is attached to thecircumferential edge 160 of the superior base plate 130 and the secondstop 176 is attached to the circumferential edge 162 of the inferiorbase plate 106. In the embodiment shown, these stops, which are designedto prevent the set screw from slipping out, take the form of thin,flexible wires that stick out over or into the threaded grooves. FIG. 15b shows a close up view of an illustrative recoil wire 176 that is insetinto the circumferential edge 162 of the inferior base plate 106. Thewires flex away from the groove as a set screw is screwed into place,allowing the set screw to be inserted into the groove unhindered,however, once the set screw is fully inserted and the face of the setscrew is flush with the circumferential edge of the base plates, thewires recoil back over or into the groove, preventing the set screw frombecoming dislodged. The recoil wires may be attached to thecircumferential edge of the base plates, as shown in the figure, or maybe inset slightly into the threaded groove. Although only two stops areshown in FIG. 12, more than two stops may be used and these may bepositioned at a variety of locations around the threaded grooves.

In an alternative design, the set screw may have a head that includesone or more screw apertures extending through its top face. Theseapertures may be aligned with tapped bores in the base plates and screwsmay be inserted through the apertures into the tapped bores in order tosecure the set screw to the base plates. A cross-sectional view of a setscrew that may be used in this manner is shown in FIG. 18. As shown inthe figure, the set screw 178 includes a tapered threaded body 180 and aflat face 182 having a plurality of screw apertures 183. The number andspacing of the apertures is not critical, provided at least one aperturemay be lined up with at least one opposing tapped bore in a base platewhen the set screw is in place. FIG. 19 a shows a disc prosthesis havingtapped bores 185 in its superior 130 and inferior base plates 106 andFIG. 19 b shows the disc prosthesis of FIG. 19 a with the set screw 178of FIG. 18 secured by four screws 187 to its superior and inferior baseplates.

In an alternative embodiment, the apertures in the face of the set screwmay be positioned such that a bone screw may be inserted through one ormore of the apertures and into a vertebra when the disc prosthesis is inplace. In this embodiment, the face of the set screw should have adiameter large enough to position the screw apertures over the vertebrawhen the disc prosthesis is in place. Alternatively, the face of the setscrew may include tabs that extend outwardly from the face of the setscrew and overlap with the vertebra when the disc prosthesis is inplace, allowing bone screws to be inserted through screw apertures inthe tabs and into the vertebra.

FIG. 20 shows a front view of a disc insert that may be used, forexample, with a disc prosthesis of the type shown in FIG. 9 or FIG. 12.FIG. 21 shows a cross-sectional view of the disc insert. This insertallows for axial loading, to provide a more natural range of motion. Thedisc insert includes a first convex surface 184, a second convex surface186, and a central collar 188. The collar extends through the discinsert and includes a flat ring portion 190 and an outer circumferentialwall 192 having a circumferential groove 194 that separates the outercircumferential wall into an upper rim 196 and a lower rim 198. The twoconvex surfaces are formed by a wall having a plurality of compressiblehelical slits 200 defined therein. As shown in the figures, the slitsare desirably disposed on the convex surfaces in at least partialoverlapping relation. The slits are compressible, such that forcesexerted by vertebrae on a disc prosthesis that includes the disc insert,are transferred along the convex surfaces through the overlappingregion, providing a spring-like characteristic.

FIGS. 22-24 show some optional features of the disc prostheses that mayhelp to more accurately mimic the natural motions of an intervertebraldisc. FIG. 22 shows a side view of the disc prosthesis of FIG. 12,including two elastic cables 202, 204 connected between the superiorbase plate 130 and the inferior base plate 106. These cables are used torestrict the rotational motion of the base plates. In FIG. 22 the cablesare depicted running substantially parallel. FIG. 23 shows analternative cable arrangement where the cables 206, 208 criss-cross. Inboth configurations, the disc prosthesis may include a second pair ofcables (not shown) disposed symmetrically on the other side of theprosthesis. If the superior base plate 130 is viewed from above as atwelve hour clock face with the twelve o'clock position corresponding tothe most posterior point of the plate in FIGS. 22 and 23, the cables aresecured to the base plates at approximately the 2:00 and 4:00 positionsand a symmetrically disposed pair of cables on the other side of theprosthesis would be secured at approximately the 8:00 and 10:00positions. However, these positions are not critical and it should beunderstood that the cables may be secured at other positions.

FIG. 24 shows a cross-sectional side view of an assembly including a cup210 adjustably mounted to a base plate 212 via a threaded stem 214. Thisassembly is similar to that shown in FIG. 1, with the exception that thecup 210 of FIG. 24 includes a flat strip 216 running through the centerof the concave surface 218 in a direction substantially perpendicular tothe anterior-posterior axis of the base plate. When the assembly isincorporated into a ball-in-socket type disc prosthesis, the convexsurface which engages the cup is able to translate along the strip. Thisdesign may be used to more accurately mimic the natural motion of anintervertebral disc.

A prosthetic vertebral assembly is shown in FIGS. 25 and 26. FIG. 25shows a front view of the prosthetic vertebral assembly 220 implantedbetween a superior vertebra 222 and an inferior vertebra 224. FIG. 26shows a cross-sectional front view of the assembly. The prostheticvertebral body includes a first intervertebral disc prosthesis 226, aprosthetic vertebral body 228 and a second intervertebral discprosthesis 230. FIGS. 27 and 28 show a cross-sectional and front view,respectively, of the prosthetic vertebral body 228. The base of theprosthetic vertebral body in this embodiment is composed of a threadedrod 232 characterized by a superior end 234 and an inferior end 236. Thethreaded rod optionally includes a central collar 237 characterized byan upper 238 surface, a lower surface 239 and a circumferential edge241. One or more holes 243 adapted to engage with a tool that grips androtates, or grips and immobilizes, the threaded rod extend into thecircumferential edge of the collar. Although the rod depicted in thefigures is a solid cylinder, it should be understood that the rod mayalso be hollow.

A superior vertically adjustable support 240 is adjustably mounted tothe superior end of the threaded rod and an inferior verticallyadjustable support 242 is adjustably mounted to the inferior end of thethreaded rod. The superior vertically adjustable support defines a firsttapped bore 244 extending into one surface thereof and the inferiorvertically adjustable support defines a second tapped bore 246 extendinginto one surface thereof. The superior and inferior verticallyadjustable supports are each characterized by an exterior surface 250,252 that faces toward a vertebra when the prosthetic vertebral body isimplanted in a patient's spine, a circumferential edge 254, 256 and aninterior surface 258, 260 that faces toward the intervertebral spacewhen the prosthetic vertebral body is implanted in patient's spine. Likethe collar on the threaded rod, the circumferential edges of theadjustable supports may optionally include one or more holes 261 adaptedto engage with a tool that grips and rotates, or grips and immobilizes,the adjustable supports.

The prosthetic vertebral body depicted in FIGS. 25 and 26 show anexample of a mechanism that may be used to lock in the height of thebody once it has been properly adjusted. This mechanism is analogous tothat depicted in FIG. 4, above. In this design, both the superior andinferior vertically adjustable supports include one or more notches 264,265 cut into their circumferential edges 254, 256 along their interiorsurfaces 258, 260. The central collar 237 of the threaded rod includesone or more notches 262 cut into its circumferential edge 241 along itsupper surface 238 and one or more notches 263 cut into itscircumferential edge 241 along its lower surface 239. A plurality oftapped holes 266 extend radially into the threaded rod above and belowthe central collar 237. As the superior and inferior verticallyadjustable supports are rotated outwardly from a position where theyrest against the collar, the tapped holes in the threaded rod becomeexposed. The tapped holes are vertically displaced from one anotheraround the circumference of the threaded rod, such that more threadedholes are exposed as the superior and inferior vertically adjustablesupports are rotated away from the collar. For example, the tapped holesmay be displaced such that one additional tapped hole becomes exposedevery time an adjustable support is rotated outwardly by an additional 1millimeter. However, other displacements are possible.

FIGS. 25 and 26 show how the notches in the vertically adjustablesupports and the collar of the threaded rod may be used to lock in theheight of the prosthetic vertebral body once it has been properlyadjusted. When a notch on a vertically adjustable support is lined upopposite and facing a notch on the collar, the pair of notches form aframe into which a tab 268, such as that shown in FIGS. 5-7, may befastened against the threaded rod using a screw 270 that engages one ofthe tapped holes 266 extending radially into the threaded rod. When thetabs 268 are in place, the vertically adjustable supports 240, 242 areunable to rotate with respect to the collar 237. The screws 270 may bealigned at a substantially right angle with respect to the long axis ofthe threaded rod 232, or may be aligned at a different angle to make itmore accessible in situ. The contour of the inner surfaces of the tabs268 may be designed to match the contour of the outer surface of thethreaded rod 232 to provide a snug fit when the tab is screwed intoplace. Although the tab and frame in FIGS. 25 and 26 are generallyrectangular in shape, it should be understood that a variety ofalternative shapes may also be employed.

In the exemplary embodiment depicted in FIGS. 25 and 26, the first andsecond disc prostheses 226, 230 each includes a first base plate 270,272 characterized by an exterior surface 274, 276 and an interiorsurface 278, 280 that defines a concave surface 282, 284. The discprostheses further include a second base plate 286, 288 that isintegrated with the one of the vertically adjustable supports 240, 242of the prosthetic vertebral body 228. The second base plate 286, 288 isalso characterized by an interior surface 294, 296 that defines aconcave surface 298, 300. The concave surfaces of the first and secondbase plates are disposed opposite one another in a substantiallyparallel relation, such that the concave surfaces of the first andsecond base plates are disposed opposite and facing one another. Thedisc prostheses each also include a disc insert 302, 304 having twoopposing convex surfaces 306, 308 and 310, 312 disposed between and incontact with the two opposing concave surfaces of the base plates. Eachof the first base plates includes a plurality of pins 310 on itsexterior surface for attaching the base plates to the superior andinferior vertebrae.

The invention has been described with reference to specific illustrativeembodiments. However it should be understood that many variations andmodifications may be made while remaining within the spirit and scope ofthe invention.

1. An intervertebral disc prosthesis comprising: (a) a first base platecomprising an exterior surface and an interior surface, the interiorsurface having a first cup disposed thereon, the first cup defining afirst concave surface; (b) a second base plate having an exteriorsurface and an interior surface, the interior surface having a secondcup disposed thereon, the second cup defining a second concave surface;the interior surface of the first base plate disposed opposite theinterior surface of the second base plate and the first concave surfacedisposed opposite and facing the second concave surface; and (c) a discinsert disposed between the first and second cups, the disc insertcomprising two opposing convex surfaces capable of articulating with thefirst and second concave surfaces of the first and second cups; whereinat least one of the first and second cups is mounted to its base platethrough a vertically adjustable support.
 2. The intervertebral discprosthesis of claim 1, wherein the vertically adjustable supportcomprises a threaded stem and the base plate defines a tapped bore, andfurther wherein the threaded stem screws into the tapped bore such thatthe distance between the cup and the base plate can be adjusted byrotating the stem in the bore.
 3. The intervertebral disc prosthesis ofclaim 1, wherein the circumferential shapes of the concave surfaces ofthe cups are ellipsoidal.
 4. The intervertebral disc prosthesis of claim1, wherein the circumferential shapes of the concave surfaces of thecups are circular.
 5. The intervertebral disc prosthesis of claim 1,wherein the first and second base plates are characterized by ananterior-posterior axis and the centers of concavity of the first andsecond concave surfaces are offset posteriorly with respect to themidpoint of the anterior-posterior axis.
 6. The intervertebral discprosthesis of claim 1, further including a plurality of cables fastenedbetween the first and second base plates for restricting the rotationalmotion of the intervertebral disc prosthesis.
 7. The intervertebral discprosthesis of claim 1, wherein the first concave surface and the secondconcave surface each comprise a flat strip running through the apex ofthe concavity.
 8. The intervertebral disc prosthesis of claim 1, whereinthe at least one cup mounted to its base plate through a verticallyadjustable support is characterized by a circumferential edge and afirst notch extends into the circumferential edge of the cup, andfurther wherein the base plate to which the at least one cup is mountedis characterized by a circumferential edge and a second notch extendsinto the circumferential edge of that base plate, such that the firstand second notches are disposed opposite and facing one another toprovide a frame.
 9. The intervertebral disc prosthesis of claim 8,further comprising a tab disposed in the frame and fastened against thevertically adjustable support.
 10. An intervertebral disc prosthesiscomprising: (a) a first base plate comprising an exterior surface and aninterior surface, the interior surface having a cup disposed thereon,the cup defining a concave surface; and (b) a second base plate havingan exterior surface and an interior surface, the interior surface havinga knob disposed thereon, the knob defining a convex surface, wherein theinterior surface of the first base plate is disposed opposite theinterior surface of the second base plate, such that the cup and theknob fit together to provide an articulating joint; and further whereinat least one of cup or knob is mounted to its base plate through avertically adjustable support.
 11. The intervertebral disc prosthesis ofclaim 10, wherein the vertically adjustable support comprises a threadedstem and the base plate to which the support is mounted defines a tappedbore, and further wherein the threaded stem screws into the tapped boresuch that the distance between the cup or knob and the base plate can beadjusted by rotating the stem in the bore.
 12. An intervertebral discprosthesis assembly comprising: (a) a first base plate comprising acircumferential edge, an interior surface, an exterior surface and afirst threaded groove extending into the circumferential edge along theinterior surface; (b) a second base plate disposed opposite the firstbase plate, the second base plate comprising a circumferential edge, aninterior surface, an exterior surface, and a second threaded grooveextending into the circumferential edge along the interior surface, thesecond threaded groove disposed opposite and facing the first threadedgroove; and (c) a threaded rod that engages the first and secondthreaded grooves of the first and second base plates.
 13. Theintervertebral disc prosthesis assembly of claim 12, further comprisinga joint disposed between the first and second base plates.
 14. Theintervertebral disc prosthesis assembly of claim 12, wherein theinterior surface of either the first or second base plate has a cupdisposed thereon, the cup defining a concave surface, and the otherinterior surface has a knob disposed thereon, the knob defining a convexsurface, wherein the concave surface of the cup and the convex surfaceof the knob fit together to form an articulating joint.
 15. Theintervertebral disc prosthesis assembly of claim 12, wherein theinterior surface of the first base plate has a first cup disposedthereon, the first cup defining a first concave surface, and theinterior surface of the second base plate has a second cup disposedthereon, the second cup defining a second concave surface, and furtherwherein the disc prosthesis system further comprises a disc insertcomprising two opposing convex surfaces disposed between the first andsecond concave surfaces.
 16. The intervertebral disc prosthesis assemblyof claim 12, further comprising at least one flexible wire extendingover at least one of the first and second threaded grooves, wherein thewire flexes out of the way when the threaded rod is screwed between thefirst and second threaded grooves and recoils back over the face of thethreaded rod when the rod has been fully screwed into place.
 17. Theintervertebral disc prosthesis assembly of claim 12, wherein thethreaded rod comprises a face defining at least one aperture thatextends over the circumferential edge of the first or second base plateand at least one of the first or second base plates comprises a tappedhole along its circumferential edge that may be aligned with the atleast one aperture.
 18. The intervertebral disc prosthesis assembly ofclaim 12, wherein the threaded rod comprises a face defining at leastone aperture that extends over a vertebra when the disc prosthesisassembly is in place in an intervertebral space.
 19. An intervertebraldisc prosthesis comprising: (a) a first base plate comprising anexterior surface and an interior surface, the interior surface having afirst cup disposed thereon, the first cup defining a first concavesurface; (b) a second base plate having an exterior surface and aninterior surface, the interior surface having a second cup disposedthereon, the second cup defining a second concave surface; the interiorsurface of the first base plate disposed opposite the interior surfaceof the second base plate and the first concave surface disposed oppositeand facing the second concave surface; and (c) a disc insert disposedbetween the first and second concave surfaces, the disc insertcomprising an exterior wall forming two opposing convex surfaces capableof articulating with the first and second concave surfaces of the baseplates, wherein the exterior wall defines a plurality of compressiblehelical slits.
 20. The intervertebral disc prosthesis of claim 19,wherein the plurality of helical slits are disposed in a substantiallyparallel relation.
 21. An adjustable intervertebral disc prosthesiscomprising an intervertebral disc prosthesis characterized by a discprosthesis height, the intervertebral disc prosthesis comprising: (a)means for simulating one or more degrees of motion of a naturalintervertebral disc; and (b) means for adjusting the disc prosthesisheight in situ.
 22. A prosthetic vertebral assembly comprising: (a) aprosthetic vertebral body comprising: (i) a base having a superior endand an inferior end, the superior end disposed opposite the inferiorend; (ii) a superior vertically adjustable support adjustably mounted tothe superior end of the base; and (iii) an inferior verticallyadjustable support adjustably mounted to the inferior end of the base;(b) a first intervertebral disc prosthesis mounted to the superiorvertically adjustable support; and (c) a second intervertebral discprosthesis mounted to the inferior vertically adjustable support. 23.The prosthetic vertebral assembly of claim 22, wherein the basecomprises a threaded rod, the superior vertically adjustable supportdefines a first tapped bore adapted to be screwed onto the superior endof the threaded rod and the inferior vertically adjustable supportdefines a second tapped bore adapted to be screwed onto the inferior endof the threaded rod.
 24. The prosthetic vertebral assembly of claim 22,wherein at least one of the first and second intervertebral discprostheses has a vertically adjustable disc height.
 25. The prostheticvertebral assembly of claim 22, wherein one or both of the first andsecond intervertebral disc prostheses comprises: (a) a first base platecomprising an exterior surface and an interior surface, the interiorsurface having a first cup disposed thereon, the first cup defining afirst concave surface; (b) a second base plate having an exteriorsurface and an interior surface, the interior surface having a secondcup disposed thereon, the second cup defining a second concave surface;the interior surface of the first base plate disposed opposite theinterior surface of the second base plate and the first concave surfacedisposed opposite and facing the second concave surface; and (c) a discinsert disposed between the first and second cups, the disc insertcomprising two opposing convex surfaces capable of articulating with thefirst and second concave surfaces of the first and second cups.
 26. Theprosthetic vertebral assembly of claim 22, wherein the base furthercomprises a central collar that divides the threaded rod into an upperthreaded section and a lower threaded section, the central collar ischaracterized by an upper surface, a lower surface and a circumferentialedge.
 27. The prosthetic vertebral assembly of claim 26, wherein thesuperior vertically adjustable support is characterized by acircumferential edge and a notch extends into the circumferential edgeof the superior adjustable support; the inferior vertically adjustablesupport is characterized by a circumferential edge and a notch extendsinto the circumferential edge of the inferior adjustable support, andfurther wherein the central collar comprises at least one notchextending into its circumferential edge along its upper surface and atleast one notch extending into its circumferential edge along its lowersurface, such that the at least one notch on the superior adjustablesupport is disposed opposite and facing the at least one notch along theupper surface of the central collar to provide a first frame and the atleast one notch on the inferior adjustable support is disposed oppositeand facing the at least one notch along the lower surface of the centralcollar to provide a second frame.
 28. The intervertebral disc prosthesisof claim 27, further comprising a first tab disposed in the first frameand fastened against the upper threaded section of the threaded rod anda second tab disposed in the second frame and fastened against the lowerthreaded section of the threaded rod.